Field of the Invention
This invention relates to color stabilized polyarylates, more particularly it relates to color stabilized polyesters. In the art the term polyarylate has come to mean the ester interchange reaction products of phenol esters of nonvicinal benzene dicarboxylic acids and aromatic polyhydric alcohols such as bisphenol A, etc.
Colorless polyarylates form color on heating in the presence of oxygen. This is well known in the art and limits the use of these compounds. Example 2 specifically demonstrates this. A. David and J. H. Golden, in the European Polymer Journal, 1968, Vol. 4, 581, demonstrate that in aromatic aryl esters degradation proceeds via a free radical process. The result of this degradation is that the colorless transparent polymer degrades to a yellow to brownish color.
For molding applications it is imperative that the molded object be colorless and therefore color stabilized polyarylates are essential to give these compounds utility. This is particularly true for aromatic polyesters which are used as engineering plastics for automobile headlights and related applications where the product has to be colorless and transparent. In the past it has been difficult to achieve this since the molding operation itself places oxygen at elevated temperatures and pressures in contact with the polyarylate thus causing its degradation and color formation. It is well established that unstabilized polyarylates usually degrade during molding if they have not already degraded as shown in Table 1.
The prior art discloses some attempts to deal with this problem thus, in the Japanese Pat. No. 74 36,752 discloses that certain polyesters can be stabilized with phosphorus acid and triaryl or trialkyl phosphites. However, the concept that epoxides and phosphates or phosphonates can be utilized to color stabilized polyesters is unknown to the art.
The preparation of some of the polyarylates stabilized according to applicant's process are disclosed in U.S. Pat. No. 3,772,389 incorporated herein by reference. Thus, aryl phenolic compounds disclosed in the aforementioned U.S. Patent are useful provided that they contain at least one hydrogen atom on the benzene ring ortho to the hydroxyl moiety of the phenol compound. Suitable aromatic hydroxyl compounds include bisphenol A, hydroquinone, resorcinol, 4',4"-dihydroxy 1,1-diphenyl methane, 4',4"-dihydroxy 1,1-diphenyl ethane, 4',4"-dihydroxy 1,1-diphenyl butane, 4',4"-dihydroxy 1,1-diphenyl isobutane, 4',4"-dihydroxy 1,1-diphenyl propane, 4',4"-dihydroxy 2,2'-diphenyl butane, 4',4"-dihydroxy 2,2'-diphenyl pentane, 4',4"-dihydroxy 6,6'-diphenyl heptane, (4',4"-dihydroxy diphenyl)(phenyl) methyl methane, (4,4'-dihydroxy diphenyl)-diphenyl) methane, 4',4"-dihydroxy 1,1-diphenylcyclopentane, 4',4"-dihydroxy 1,1-diphenylcyclohexane, 4',4"-dihydroxy-3',3"-dichloro-2,2-diphenylpropane, 4',4"-dihydroxy-3',3", 5',5"-tetrachloro- 2,2-diphenylpropane, 4',4"-dihydroxy-3',3",5',5"-tetrabromo-1,1-diphenylcyclohexane, 4',4"-dihydroxy-3',3"-dimethyl-2,2-diphenyl propane, 4,4'-dihydroxy diphenyl, 4,4'-dihydroxy diphenyl oxide, 4,4'-dihydroxy diphenyl sulphide, 4,4'-dihydroxy diphenyl sulphoxide, 4,4'-dihydroxy diphenyl sulphone, 2,2'-dihydroxy diphenyl, 4,4'-dihydroxy-2',2,6,6'-tetramethyl diphenyl, 4,4'-dihydroxy benzophenone, 2,4'-dihydroxy biphenyl, 1,7-dihydroxy naphthalene, 1,6-dihydroxy naphthalene, 1,5-dihydroxy naphthalene, 2,2'-dihydroxy benzophenone, 2,4'-dihydroxy benzophenone, 2,6-dihydroxy naphthalene, 2,7-dihydroxy naphthalene, 1,5-dihydroxy anthracene, 1,8-dihydroxy anthracene, 1,5-dihydroxy anthraquinone, ##STR1##
Suitable aromatic carboxylic acids include, among others, isophthalic acid, terephthalic acid, 2,6 and 2,7 naphthalene dicarboxylic acid, etc. Generally, any dicarboxylic acid can be utilized to prepare polyesters which has carboxyl groups directly attached to the same or different aromatic rings. The rings may have one or more substituents inert in the polymerization reaction. The catalysts useful in the polymerization include sodium hydroxide and sodium dithionite, potassium hydroxide and triphenylphosphite, lithium hydroxide, tri decyl phosphite, aluminum phosphate, sodium phosphate (mono basic), aluminum sulphate, boric anhydride, sodium phosphate (dibasic), sodium pyrophosphate, potassium phosphate, sodium borate, sodium borohydride, boric anhydride, dibutyltin oxide, zinc borate, calcium acetate, and combinations of the above such as lithium hydroxide plus tridecylphosphite, potassium hydroxide plus boric acid.
It has been discovered that polyesters prepared in any conventional manner disclosed herein or in the prior art can be color stabilized by the addition of mixtures of either phosphates or phosphonates with epoxide having a boiling point above 250.degree. C. suitably having a boiling point in the range of 250.degree. C. to 400.degree. C. Usually the color stabilizers comprise about 0.005 to 5% by weight of the aromatic polyester, preferably 0.01 to 0.5%. Preferred phosphates include tributyltrithiophosphate and 2,6-di-t-butylphenyl phosphate and preferred phosphonates include 0,0-di-n-octadecyl-3,5-di-tert-butyl-4-hydroxy benzyl phosphonate.
Useful epoxides include all epoxides having a boiling point above 250.degree. C. Particularly useful epoxides are 3,4-epoxycyclohexylmethyl-3,4-epoxy cyclohexane carboxylate, 3,4-epoxy 6-methyl cyclohexylmethyl-3,4-epoxyl-6-methylcyclohexane carboxylate, 4(3,4-epoxyl-6-methyl cyclohexylmethyl) butyl 3,4-epoxyl-6-methyl cyclohexane carboxylate, di-3,4-epoxyl-6-methyl cyclohexylmethyl adipate, bis-phenol-A diglycidyl ether, bis epoxycyclohexyl adipate, epoxidized edible oils, the preferred epoxy compound is di-3,4-epoxy-6-methylcyclohexyl adipate.
The color stabilization is suitably illustrated on a polymer prepared from bisphenol A and isophthaloyl chloride and terephthaloyl chloride. In this illustation suitably 68.4 parts by weight of bisphenol A are added to a solution of sodium hydroxide (25.2 parts by weight), deoxygenated water (14.50 parts by weight) together with sodium dithionite (0.2 parts by weight) and benzene triethyl ammonium chloride (0.3 parts by weight) in a suitable blender. To this are added terephthaloyl chloride (30.5 parts by weight) and isophthaloyl (30.6 parts by weight) in methylene chloride under medium speed stirring. After stirring for a short while, the polymer was precipitated by adding the solution to an excess of acetone and the solid precipitate was washed with acetone. One gram of the polymer was dissolved in methylene chloride (10 ml) and 0.05 ml of a 1% solution of 2,6-di-t-butylphenylphosphate and 0.05 ml of a 1% solution of a dicyclo diepoxy carboxylate was also added. A film was cast from the stabilized solution and heated to 200.degree. C. for six days. The polyester polymer remained colorless and retained all of its physical properties. This demonstrates that the stabilized compositions are useful in inhibiting the color formation of polyester when exposed to oxygen at elevated temperatures and makes them useful as engineering plastics for automobile headlights and related application.